1. Field of the Invention
This invention relates generally to telecommunication systems and assemblies, and more particularly to a chirped fiber acousto-optic bandpass filter.
2. Description of the Related Art
An important function in the telecommunication industry is signal switching. The switching can be performed either electronically or optically. In past years, this switching was accomplished through electronic means. However, with the increasing demand for lower cost, higher switching speeds, lower power consumption, and lower optical losses, optical switching is becoming more commonplace. There are two types of optical switches currently used, wavelength insensitive optical switches and wavelength sensitive optical switches. The wavelength insensitive optical switches are typically broadband fiber-to-fiber switches used to redirect all the traffic from one optical fiber to another. Because the switching process is either thermo, electro-optic, or mechanical, the switching speed is slow but satisfactory. However these switches do not satisfy the requirements for low cost, high reliability, and low optical insertion loss.
The wavelength sensitive optical switches are needed for wavelength division multiplexed (WDM) signals because the wavelength separation between channels is small. A narrow optical band of traffic carried by a specific wavelength of a multi-wavelength signal may need to be separated from the rest of the traffic. A wavelength-sensitive optical switch can perform this function optically at considerable cost savings. Existing wavelength sensitive optical switches are usually bulky, have high power consumption, and high optical insertion losses. For instance, in a previous patent by this inventor entitled Tunable Optic Fiber Bandpass Filter Using Flexural Acoustic Waves, U.S. Pat. No. 6,151,427, an acousto-optic bandpass filter was described, however that invention uses a core block that introduces significant optical insertion losses, added complexity, and is costly to manufacture. The present invention does not require a core block component, thereby negating these problems and simplifying the architecture. Other acousto-optic filters include xe2x80x9cAcousto-optic Filter,xe2x80x9d U.S. Pat. No. 6,233,379 by Kim et al, which is hereby incorporated by reference. The filter described performs the function of a band-stop filter and can select a limited number of optical bands (channels) simultaneously but does so at the cost of increased power consumption for each band selected to the limit of the acousto-optic generator. The current invention eliminates all the optical bands (channels) simultaneously and can then select one or more channels to pass through the filter, thus performing a bandpass operation.
In consideration of the problems detailed above and the limitations enumerated in the partial solutions thereto, an object of the present invention is to provide an improved chirped fiber acousto-optic bandpass filter that does not require a core-block and uses less electrical power.
Another object of the present invention is to provide a chirped fiber acousto-optic bandpass filter with multiple acoustic signals that have individual controllable strengths and frequencies.
Yet another object of the present invention is to provide a broadband cladding mode coupler to efficiently couple all the optical traffic from a fiber core mode to a fiber cladding mode for later selection of individual optical channels.
Yet a further object of the present invention is to provide an all-fiber chirped acousto-optic bandpass filter that includes an optical fiber with a core and a cladding where the strength or the magnitude of an optical signal coupled from the cladding to the core is changed by varying the amplitude of the electrical sinusoidal frequency applied to an acoustic wave generator.
Yet another object of the present invention is to provide a narrow-band core mode coupler that efficiently re-couples a particular optical channel from a cladding mode to a core mode and tuned by tensioning the fiber within the narrowband core mode coupler.
In order to attain the objectives described above, according to an aspect of the present invention, there is provided a tension-tuned acousto-optic bandpass filter whereby one or more bands of optical wavelengths may be selected for further transmission. In this device, all light within the optical bandwidth of operation is first coupled from the core mode of a optical fiber to a specific cladding mode within a first acousto-optic interaction region, a chirped broadband cladding mode coupler, where a selected RF frequency of a flexure wave, induced by a first acoustic wave amplifier, acting on a region of the optical fiber that has been chirped, couples all light within the optical bandwidth of operation from the core mode of the optical fiber to a specific cladding mode. These cladding mode lightwaves then enter a second acousto-optic interaction region, containing a narrow-band core mode coupler, where traveling flexure waves, induced by a second acoustic wave amplifier, re-couple selected bands of wavelengths back into the core. The particular bands of wavelengths to be re-coupled into the core can be tuned by adjusting the tension of the fiber within the narrow-band core mode coupler of the second acousto-optic region. The second acousto-optic interaction region is isolated from the first acousto-optic interaction region by an acoustic absorber to limit acoustic interaction between the acousto-optic interaction regions.